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Chapter 11
INTRODUCTION TO GENETICS
A. MENDEL
Early Life
 Abbot Gregor Johann Mendel (1822-1884, Johann Mendel) was a monk of the
Augustinian order n Brunn, Austria.
 After failing to qualify as a high school biology teacher, he joined the monastic
order.
 In 1851, his superiors sent him to the University of Vienna for 2 years.
Work
 Mendel began experiments on the effects of crossing different strains of the
common garden pea.
 He used mathematics to examine his results.
 Mendel chose seven different pairs of contrasting pea traits with which to work.
Note: D=dominant trait and R=recessive trait.
a.
Seed form: round (D) or wrinkled (R).
b.
Color of seed: yellow (D) or green(R).
c.
Color of flower: Purple (D) or white (R).
d.
Color of unripe seed pods: green (D) or yellow (R).
e.
Shape of ripe seed pods: inflated (D) or constricted between seeds (R).
f.
Length of stem: short (9-18 inches) (R) or tall (6-7 feet) (D).
g.
Position of flower: axial (in axial of leaves) (D) or terminal (at the ned of
the stem) (R).
Vocabulary
 Genotype-Combination of an organisms genes.
 Phenotype-Combination of visible traits.
 Allele- forms of a gene that code for a particular trait.
 For example, W is a gene that codes for a widows peak and w codes for a straight
hairline.
 Although they both code for the hairline they are two different alleles, alternative
forms of the gene that code for the same trait.
 Homozygous; “true breeding strain” it has only one kind of allele for the trait.
 Heterozygous, “ hybrid strain” has alleles for both traits.
 F1- first generation
B. PRINCIPLE OF DOMINANCE
When crossing true–breeding round with a true-breeding wrinkled strain Mendel
discovered the following:
 1/4 of the total F2 were round and true breeding (homozygous).
 1/2 of the total F2 were round and not true breeding (heterozygous).


1/4 of the total F2 were wrinkled and true breeding (homozygous).
Mendel termed the trait that appeared in the F1 generation the dominant trait,
and the trait that failed to appear in the F1 generation the recessive trait.
Mendel used algebraic symbols to represent what was happening. He let upper
case letters (A) represent the dominant factor and lower case letters (a) represent
the recessive factors. The heterozygous had both factors (Aa).
If the heterozygous plant was ‘Aa’, the plant received ‘A’ from one parent and ‘a’
from the other parent. If a plant was homozygous dominant, it obtained one ‘A’
from each parent. If a female parent was ‘AA’ and the male parent was ‘aa’,
Mendel reasoned that the female could only give ‘A’ to the offspring and that the
male could only give ‘a’ to the offspring. All offspring would be ‘Aa’.
If the ‘Aa’ offspring were allowed to mate, then 1/4 of their offspring would be
‘AA’, 1/2 would be ‘Aa’ and 1/4 would be ‘aa’.
P1 = AA x aa
F1 = Aa x Aa
F2 = AA 2Aa aa
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Mendel’s conclusions:
The seven characteristics were controlled by transferable factors.
The factors came in two forms: dominant and recessive.
Today, we call these transferable factors genes.
Every heterozygote (hybrid) had 2 different copies of the factor controlling each
character – one from each parent.
The dominant factor determined the appearance of the plant (phenotype).
C. MENDEL’S FIRST LAW: THE LAW OF SEGREGATION
 The two alleles for a trait separate (or segregate) when gametes are formed.
 When a heterozgote reproduces, its gametes will be of two types in equal
proportions.
 Either the gamete will have ‘A’ or ‘a’.
Aa  1/2 A or 1/2 a
 Early in the 20th century, these relationships were put into graphic form by
Reginald Punnett.
 The forms are called punnett squares.
 Each little square represents a possible offspring.
 Above the squares are the parents’ gametes
Parental Gametes 
Possible offspring
Genotypess 
If Mom Aa and Dad Aa decided to have a child, what are the possible genotype of
the offspring and their probabilities?
Mom’s gametes:
_______ or _______
Dad’s gametes:
_______ or _______
Construct a punnett square:
Parental Gametes
?
?
?
There is a __ in 4 or ___% chance of the child being AA
There is a __ in 4 or ___% chance of the child being Aa
There is a __ in 4 or ___% chance of the child being aa
?
D. MENDEL”S SECOND LAW OF INDEPENDENT ASSORTMENT
 In gamete formation each pair of factors segregates independently of other pairs
of factors.
 In chromosome terms, each pair of homologs segregate independently of every
other pair in Meiosis I.
 Pairs of alternative traits behaved independently.
 This is because maternal and paternal chromosome pairs line up and separate
during meiosis.
 If an organism is heterozygous at two unlinked loci (two genes for two traits are
on different pairs of chromosomes), each locus will assort independently of any
other.
Crosses
Mendel first studied one characteristic at a time. He next crossed plants with
two sets contrasting traits, e.g. a plant that is true breeding for both round and
yellow seeds with a true breeding plant with wrinkled and green seeds.
The F1 seeds would all be round and yellow. A=round, a=wrinkled, B=yellow,
b=green.
P1=AABB X aabb
F1=AaBb
If we cross the F1 together to get an F2, then how can we predict the F1
offspring?
We first have to determine the parental gametes of the F1, and then place these
gametes in a Punnett square.
Two parents AaBb mate; how can we determine the gametes?
We know that the parents will give their offspring one A (or a) gene and one
B (or b) gene. What are the different combinations?
The law of independent assortment is applied in determining the parental
gametes.
FOIL
AB,Ab,AB, or ab with equal possibilities. There are four possible gametes
from each parent.
Below is the Punnett square.
Gametes
AB
Ab
AB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
Aabb
Aabb
AaBb
Aabb
AB
AaBb
AaBb
AaBB
AaBb
ab
AaBb
Aabb
AaBb
Aabb
From this, one can determine the phenotypes.
E. DOMINANT RELATIONSHIPS
1. Lethal recessive
Homozygous recessive organisms cannot survive.
2. Partial dominance or Incomplete Dominance
The heterozygote is intermediate between the phenotypes of the two
homozygotes, and not exactly like either one of them. Snap dragons
r=red, w=white
Rr x ww  rw = pink rw x rw = 1 red, 2 pink, 1 white
Sometimes the phenotype of the heterozygote is not simply a blending o
compromise between the two homozygotes, but has unique features of its
own. E.g. Palomino horses are heterozygous for coat color. Crosses
between two palominos result in brown, 2 palominos, and 1 white foal.
3. Codominance
Sometimes one homozygote will show one phenotype, the other
homozygote will have a different phenotype, and the heterozygote will
show both phenotypes.
Blood types. AA x BB  AB
Neither allele is recessive,
both are equally dominant
4. Multiple Alleles
There are more than two alleles possible at a single locus. For example,
researcher have discovered 37 different alleles for eye color at one locus in
fruit flies. Anyone individual has, at most, two different alleles of the 37
people.
F. GENE INTERACTIONS AND THE MODIFIED MENDELIAN RATIOS
We have talked about Mendel and his peas. We have learned that if we cross
two individual that are heterozygous (AaBb), we get a 9:3:3:1 phenotypic
ratio for the offspring. This holds true only if the two pairs of alleles act
independently. For example, this does not work out for coat color of mice. At
one locus, B is dominant over b. BB and Bb mice are black and bb mice are
brown.
BB x bb  Bb = black mice
Bb x Bb  3/4 black mice, 1/4 brown mice.
At another locus, there is a gene C which is dominant over c. CC and Cc mice
can make pigment. cc mice cannot make pigment, they are albinos.
1. Epistasis
Msking of a trait determined by one pair of genes by the actions of another
pair of genes. If we cross pure-breeding brown mice (Ccbb) with truebreeding white mice (ccBB) we get all black mice (ccBb). A cross
between these F1 black mice result in: CcBb x CcBb = 9 black, 4 white, 3
brown mice. The Cc locus is epistatic to the Bb locus.
Gametes
CbCb
cB
Cb
Cb
CB
CCBB
CCBb
CcBB
CcBb
Cb
CCBb
Ccbb
CCBb
Ccbb
CB
CCBb
CcBb
CcBB
CcBb
cb
CCBb
CCbb
CcBb
ccbb
2. Pleiotropy
Pleitropy occurs when a single gene can affect more than 1
characteristic (e.g. coat color in cats: white cats often have blue and eyes
and are deaf).
G. THE GENETICS OF SEX
 In 1910 Thomas Hunt Morgan began breeding experiments with the fruit fly
Drosophila Melanogaster.
 Moran found a single male with white eyes and was convinced that this was
due to a mutation, since red is the normal eye color.
 Morgan took the white-eyed male and mated it with some red-eyed sisters.
 The F1 generation among themselves. Of the flies in the F2 generation: 1/4
were red eyed males, 1/2 were red eyed females and 1/4 were white eyed
males.
 All the white eyed flies were male.
 Morgan crossed the white eyed females with red eyed males.
 The resulting flies were all white-eyed males and all red eyed females.
 Morgan surmised that the gene carrying the white eye color was located on
the sex chromosome, specifically on the X chromosome.
H. SEX LINKAGE IN HUMANS
 Several recessive traits are carried on the X chromosomes; many cause
abnormalities.
Colorblindness: The most common type is red/green colorblindness. To be
affected, a man only needs to receive one recessive gene from his mother (on the X
chromosome) since Y is not allelic to X and cannot mask the recessive allele.
Affected women must receive the recessive gene from both parents.
I. CHROMOSOMAL ALTERATIONS
Alterations of Chromosome Number
 Nondisjunction occurs when homologous chromosomes do not separate
properly during meiosis I or the sister chromosomes fail to separate in
meiosis II.
 The result is an abnormal chromosome number, called aneuploidy (2n+1 or
2n –1).
Polyploidy
 More than two complete chromosome sets, for example, Triploidy: 3n and
tetraploidy: 4n.
 Polyploidy occurs when there is nondisjunction of a complete set of
chromosomes.
 Down’s syndrome is polyploidy of the 21st chromosome
Alterations of Chromosome Structure
 The breakage of a chromosome can lead to a variety of rearrangements
affecting the genes of the chromosome.
 Fragmennts without centromeres are usually lost when the cell divides.
 The chromosome from which the fragment originated will be missing
certain genes (deletion). In other cases, the fragment may join to the
homologous chromosome (duplication). The fragment may reattach to the
original chromosome inverted (inversion). The fragmen may attach to a
nonhomologous chromosome (translocation).
 During the crossing over process, some chromatids break at different
places, and one partner gives up more genes than it receives.
Examples of the types of chromosomal anomalies
The following are results of nondisjunction because of irregular meiosis:
An extra autosomal chromosome (e.g. trisomy 21 (Down’s
Syndrome), Edwards syndrome
b.
An extra sex chromosome (e.g. XYY, Klienfelters Syndrome,
and Triple X syndrome)
c.
Missing a sex chromosome (e.g.. Turners Syndrome, XO)
The following are not results of nondisjunction:
a.
d.
Deletion of a piece of an autosomal chromosome (e.g. crit du
chat).
e.
Translocation: piece of one chromosome is added to a nonhomologous chromosome (e.g. translocation 21 to 14, one
cause of Downs Syndrome).
J. HUMAN GENTICS AND CHROMOSOMES
The number of chromosomes in the human species is 46; 44 autosomes and two
gender (sex) chromosomes. A graphic representation of the chromosome present
in the nucleus of a cell is known as karyotype. From a karyotype, we can
determine the number, size, and shape of the chromosomes as well as identify the
homologous pairs. Sometimes, it is difficult to distinguish between similar looking
chromosomes. However, the chromosomes can be stained to show a banding
pattern. The chromosomes with the similar banding patterns are homologous
chromosomes.